This invention relates to manufacturing methods for high-density magnetic data-storage media. Other embodiments of the invention relate to high-density magnetic data storage media in general, and to high-density magnetic data storage media manufactured in accordance with the aforementioned method.
Magnetic recording technology has been driven by a strong demand for faster access speeds and for higher data storage density. To meet this demand, the areal density (i.e. the number of bits that can be stored per unit area) of magnetic hard disk data storage devices (HDDs) has been increasing rapidly year on year since 1991.
Recently, companies such as Read-Rite and Fujitsu have demonstrated that HDDs can be constructed that provide storage densities of 50 Gbit/in2 and above, and it therefore seems likely that an areal density of 200-300 Gbit/in2 will be achievable within a few years.
However, it has been predicted that current media will encounter a physical limit which will prevent, or at least make very difficult, the manufacture of high-density magnetic storage media with an areal density of over 200-300 Gbit/in2.
Currently used thin film magnetic recording media typically consist of a plurality of small, single-domain magnetic grains, which are magnetically isolated from one another. For an acceptable media signal-to-noise ratio (SNR), each recording bit must contain a number of magnetic grains. Typically several tens to several hundreds of magnetic grains in each written bit are used, with each grain having a diameter in the range of 10-20 nm or so.
To increase the storage density beyond that provided by these current media, the size of the multigrain bits (and therefore each grain in each bit) must be reduced yet further. However, if the grain size is too small, the magnetisation applied to the bit cannot be retained against thermal decay due to the very small energy barrier height. As a consequence, applied magnetisation would be able to switch easily, and thus recorded data could be lost. This phenomenon is the so-called xe2x80x9csuperparamagnetismxe2x80x9d or xe2x80x9cthermal-instabilityxe2x80x9dlimit, which is the root cause of the physical areal density limit of current HDDs.
To avoid this limit it has recently been proposed to use patterned media in high-density magnetic storage media with an areal density of from 100 Gbit/in2 to tens of Tbit/in2.
The term xe2x80x9cpatterned mediaxe2x80x9d is used generally in the art to refer to media that consist of regular arrays (i.e. patterns) of discrete magnetic elements, which can each store one bit of data. Such bits are arranged periodically to be synchronised with the signal channel. In the simplest of these proposals the elements have only a single axis of magnetisation that is interpreted as a binary 1 or 0, and in such a situation the storage density of the HDD is then equal to the surface density of the elements.
In patterned media devices, each discrete element is magnetically isolated from other elements, and inside each discrete element individual polycrystalline grains are strongly exchange-coupled so that they behave more like a larger single magnetic grain. These xe2x80x9csingle-domainxe2x80x9d magnetic elements can be made of polycrystalline materials as well as single crystal and amorphous materials.
Because the superparamagnetism limit applies to the whole single bit (because the individual grains behave more like a larger single grain), and not to each of the many grains (as it does in a continuous multigrain bit of a conventional medium), the volume and switching energy for the single-element bits in patterned media are much larger than that of single grains in conventional media. This allows a significant reduction in bit size. The minimum volume of the discrete element will still be determined by the superparamagnetic limit, but the size of the element could be as small as a few nanometers, depending on the magnetic properties of the materials. A reduction in element size to in the region of a few nanometers would give the patterned medium all areal density as high as tens of Tbit/in2.
Another advantage of these so-called patterned media is that the SNR for the read head is increased because random Nxc2xd noise associated with acceptable numbers of decoupled grains within each continuous multigrain bit is no longer applicable, and because noise associated with irregular or zigzag transitions typical of continuous thin film media is reduced.
However, one problem with patterned media or the like is that their areal density implies that both the linear bit density (bits per inch) and the track density (tracks per inch) must be in the deep sub-micrometer range, and as a result they cannot currently be cheaply manufactured using mass-production techniques.
Conventional lithographic methods either are too time-consuming or do not handle the small structure sizes needed. For example, electron beam lithography is too slow to cover large areas, and laser interferometric lithography is wavelength limited to areal densities of 100 Gbit/in2 or less by currently available short-wavelength lasers.
A major challenge is therefore to find a low cost nano-pattern generation technique that would enable these patterned media to be cheaply mass-produced.
Recently, several new techniques such as nanoimprint lithography (as utilised in U.S. Pat. Nos. 5,772,905, and 5,956,216), self-assembly using chemically monodispersed nanoparticles (as described in U.S. Pat. No. 6,162,532), and nano-templates formed from self-assembled diblock copolymer thin films (see Nikkei Electronics Asia, November 2000 issue) have been employed to fabricate prototype patterned media. However, these techniques, whilst holding some promise, are not well proven at present.
It is therefore an aim of the present invention to provide an alternative technique for the manufacture of nano-pattern media. In pursuit of this aim it has been observed that periodic ripple structures with sub-micrometer to nanometer width can be formed on semiconductor and metal surfaces by ion bombardment at off-normal-incidence angles (see S. Rusponi et al xe2x80x9cScaling Laws of the Ripple Morphology on Cu(110)xe2x80x9d, Phys. Rev. Lett., 81, 4184 (1998); and R. M. Bradley et al xe2x80x9cTheory of Ripple Topography Induced by Ion Bombardmentxe2x80x9d, J. Vac. Scl. Technol. A 6, 2390 (1988)xe2x80x94the contents of which are incorporated herein by reference), and recent results show that it is possible to form ordered crystalline dots of about 35 nm in diameter in a regular hexagonal lattice on GaSb surfaces by ion bombardment at normal incidence angle (see S. Facsko et al, xe2x80x9cFormation of Ordered Nanoscale Semiconductor Dots by Ion Sputteringxe2x80x9d, Science 285, 1551 (1999) xe2x80x94the contents of which are incorporated herein by reference). Generally speaking, these self-ordering nanostructures on semiconductor or metal surfaces are caused by the interplay between roughening due to sputtering, and smoothing because of surface diffusion/viscous flow.
It is a general object of the present invention to use these observed patterning phenomena to provide a method for fabricating high-density magnetic data storage media.
In accordance with a preferred embodiment of the present invention, there is provided a method of fabricating a high-density magnetic data-storage medium, the method comprising the steps of:
(a) forming a plurality of nanodots of non-magnetic material in a regular array on a surface of a substrate, said array being notionally dividable into a plurality of clusters that each comprise a plurality of nanodots, wherein each nanodot of a said cluster overlaps with neighbouring nanodots of that cluster to form a well between them;
(b) depositing magnetic material onto said substrate to at least partly fill the wells of each cluster; and
(c) removing material from the substrate to reveal a regular array of wells filled with magnetic material, each of said wells being separated from neighbouring wells by non-magnetic material.
In the context of the present invention, a high-density magnetic data-storage medium is defined as one with an areal density of at least 100 Gbit/in2; and a nanodot is defined as a projection or dot with dimensions that are small enough to enable an areal density of this order to be achieved (i.e. generally in the order of tens of nanometers or less).
Preferably, the wells formed as a result of step (a) are at least substantially free of said non-magnetic material.
The magnetic material can be deposited in step (b) as a layer that fills the wells and extends over the non-magnetic nanodots.
Other details of the preferred embodiment are set out in the dependent claims, and in the following description.
By employing the teachings of embodiments of the invention, each discrete magnetic element can be separated from other elements by nonmagnetic materials so that the likelihood of exchange interactions taking place between neighbouring elements is reduced.
In one highly preferred embodiment, GaSb (Gallium Antinomide) wafers or epitaxy-grown GaSb films on nonmagnetic substrates are ion sputtered to obtain ordered arrays of nanodots (a pattern). The pattern is then transferred into deep wells (preferably right down to the substrate) by reactive ion etching with a thin layer of aluminum as protection deposited on the top of the nanodots and the overlapping regions between nanodots, before etching begins. Finally, magnetic materials are grown into the wells, for example by electrodeposition, sputtering, or evaporation, and the top surface of the medium is then polished to reveal isolated areas of magnetic material, and subsequently preferably coated with an abrasion-resistant material like carbon.
In very general terms, the scope of the present invention extends to a method of fabricating a high-density magnetic data storage medium comprising the steps of: forming a plurality of non-magnetic nanodots on a substrate (for example by utilising one of the aforementioned methodsxe2x80x94particularly the method described in the aforementioned Facsko paper) so that each of said nanodots overlaps with its neighbours to form wells; depositing magnetic material into said wells, and removing material to reveal a plurality of magnetic-material-filled wells separated from one another by non-magnetic material.